Transmitter



Patented June 14, 1938 UNITED STATES TRANSMITTER William Theodore Ditcham, London, England, assignor to Radio Corporation of America, a corporation of Delaware Application March 25, 1936, Serial No. 70,751 In Great Britain March 27, 1935 3 Claims.

This invention relates to modulated carrier wave transmitters, for example, to radio transmitters.

The object of the invention is to provide an 5 improved modulator arrangement of high efficiency and small distortion and which shall be practically convenient and relatively cheap and easy to construct.

A well known form of modulator arrangement-sometimes termed a class B modulatorcomprises two or more modulating tubes which are utilized to modulate a high frequency (carrier) tube stage, said modulating tubes being in phase opposition both as regards their anodes and grids and having their grids so biased that approximately zero anode current flows therein in the absence of applied modulated potentials, the anode circuits of the modulating tubes being coupled to the anode circuit of the high frequency (carrier) tube by means of a special known form of push-pull transformer sometimes termed a class B transformer. This and similar so-called class B modulator arrangements offer the advantages of giving considerable economy in power output as compared with many other known modulator systems, but class B push-pull transformers are somewhat expensive and moreover introduce difficulties in obtaining rectilinear modulation and a wide frequency response of substantially constant amplitude.

The present invention provides improved modulator arrangements presenting the advantages of the known class B modulators above referred to, but without their disadvantages and without requiring any push-pull or class B output transformer.

According to this invention modulation of the oscillatory energy in a carrier wave stage is cffected by means of two modulating stages one of which is included in a circuit in parallel with the carrier wave stage at which modulation is to be effected and the other of which is included in a circuit in series therewith, the modulating potentials being applied to the two modulating stages in such manner that each is responsive only to a different half wave of the input modulating potential waves, the whole arrangement being such that the parallel connected modulating tube is employed to effect modulation in one direction and the series connected modulating tube is employed to effect modulation in the other.

The high frequency stage at which modulation is effected may be either an oscillator stage or a carrier frequency amplifier stage, and it may comprise one or more tubes while, similarly, the two modulating stages may be constituted each by a single tube or each by a plurality of tubes.

The invention is illustrated in the accompanying drawing which shows diagrammatically one embodiment thereof.

Referring to the drawing the stage at which modulation is to be effected is constituted by a high frequency amplifier tube shown as a triode 2 to whose control grid carrier frequency oscillations are applied from a suitable source represented by the rectangle CF, e. g., through a condenser as illustrated. The said grid is connected to the cathode of the high frequency valve through a grid resistance 3 and a tuned output circuit LC feeding directly or indirectly into a utilization device, such as an aerial A, is coupled or connected between the anode and cathode of the high frequency tube 2. The anode of the high frequency tube is connected to the positive terminal of an anode potential source, e. g., a direct current generator 6, through a suitable carrier frequency choke l0 and the cathode of the said tube is connected to the negative terminal of the said source 6 through a similar carrier frequency choke l2 in series with a low frequency (preferably iron cored) choke L. The junction point of the choke L with the choke I2 is connected to the cathode of a parallel connected modulating tube I4, the anode of this tube being connected to the positive terminal of the anode potential source 6. The cathode of the parallel connected modulating tube is connected to the anode of a modulating tube IS, the cathode of which is connected to the negative terminal of a second source 8 of direct current potential, e. g., a second direct current generator. The source 8 has its positive terminal connected to the negative terminal of the source 6. Thus the impedances between the output electrodes of tubes 2 and I6 are in series with respect to the modulation frequencies, while the impedance of tube 2 is in a direct current series circuit including the impedance of IS and L in parallel. The negative terminal of the source 8 may be earthed. The grid circuit of the parallel connected modulating tube I4 includes the secondary of a modulation input transformer Tl in series with a suitable bias battery I8 and the grid circuit of the series connected modulating tube l6 also includes the secondary of a modulation input transformer T2 in series with a suitable bias source 20. The primaries of the two modulation input transformers Tl, T2, are connected in series with one another across the source of modulating potentials. The two modulating tubes l4, l6, are so biased that no anode current fiows in either tube in the absence of modulation potential input and the relative phase of the modulation potentials applied to the said tube grids are so chosen that one tube responds only to positive modulation half waves and the other only to negative half waves. Thus modulation of the high frequency tube in an upwards direction is effected by one modulating tube, e. g., the parallel connected modulating tube I4, while modulation in the downwards direction is effected by the other tube, e. g., the modulating tube l6 connected as described above.

With the above described arrangement the current passing through the modulating tubes will, in the absence of modulation potentials, be small in value and the normal current required for the carrier will pass through the series circuit including the high frequency tube 2 and the low frequency choke L. When modulation potentials are applied, the grid of the series modulating tube [6 will become less negative for half waves of modulation in the appropriate direction and therefore the current through the high frequency tube 2 will increase, while for the other half waves of modulation the grid of the parallel connected modulating tube I4 will become less negative and the current through the high frequency tube 2 will accordingly be decreased.

I claim:

1. A carrier wave modulation circuit arrangement comprising a carrier frequency tube stage, a source of direct current potential connected to apply anode potential between the anode and cathode electrodes of said tube stage, a low frequency choke interposed in said connection between the negative terminal of said source and said cathode electrode, a modulating tube stage having its anode to cathode impedance connected in a circuit extending from the anode electrode of said carrier tube stage to the end of said low frequency choke remote from said source, a second modulating tube stage having its anode electrode connected to said end of said low frequency choke and its cathode electrode connected to the negative terminal of a second source of direct current potential whose positive terminal is connected to the negative terminal of the first mentioned source, means for taking modulated carrier energy from the anode circuit of the carrier frequency tube stage, means for confining carrierenergy to the carrier frequency circuits of said carrier frequency stage, and means for applying modulating potentials to the grid circuits of both of said modulating tube stages, said potentials being applied in such sense and said modulating stages being so biased that each of said modulating stages is responsive only to a different half wave of the input modulating waves.

2. In a signalling system, a carrier wave amplifying tube, a first modulator tube, and a second modulator tube, each of said tubes having an anode, a cathode and a control grid, means for applying carrier wave oscillations to the control grid and cathode of said amplifying tube, a circuit connecting the impedances between the anode and cathode of said amplifying tube and said first modulator tube in parallel, a source of direct current potential also connecting the impedances between the anode and cathode of said amplifying tube and said second modulator tube in a series modulation circuit, a low frequency reactor connecting the junction point between the anode to cathode impedances of said amplifying tube and said second modulator tube to a point on said source of direct current potential to regulate the respective potentials supplied by said source across saidinter-electrode impedances during operation, and a source of modulating potentials coupled in phase displaced relation to the control grids and cathodes of said first and second modulator tubes.

3. In a carrier wave modulation system, a carrier frequency stage and two modulating stages, each stage comprising a tube having output electrodes, a circuit connecting the impedance be-* tween the output electrodes of one of said modulating stage tubes in parallel with the impedance between the output electrodes of the carrier wave stage tube, means connecting the impedance between the output electrodes of the other modulating stage tube and the impedance between the output, electrodes of said carrier wave stage tube in a series modulation circuit, a source of direct current potential connected to the terminals of said last named circuit, a low frequency reactor connecting the junction point between the tube impedances of said series modulation circuit to a point on said source of direct current potential, and means for applying modulating potentials to the two modulating stage tube impedances in phase displaced relation whereby the parallel connected modulating stage tube is employed to effect modulation of the carrier wave in one di rection and the modulating stage tube in the series modulation circuit is employed to effect modulation of the carrier wave in the other direction.

' WILLIAM THEODORE DITCHAM. 

